High molecular weight linear thermoplastic polyesters, such as poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) are well known as film and fiber-formers and they are provided by methods outlined in Whinfield et al, U.S. Pat. No. 2,465,319, and Pengilly, U.S. Pat. No. 3,047,539, and elsewhere.
The polyesters have not been exploited until very recently as molding and extrusion resins, however, because the most widely available member of the family, poly(ethylene terephthalate) crystallizes so slowly from the melt that three-dimensional parts molded from it, in conventional molding cycles, are brittle in thick sections.
This problem has been overcome to some extent by providing careful molecular weight control in the production of poly(ethylene terephthalate); by including nucleating agents, to hasten crystallization; or by using unconventional, long, two-step molding cycles.
With attention to the above-enumerated details, poly(ethylene terephthalate) molding compositions can be provided, including reinforced and flame retardant embodiments, but the compositions are expensive to produce.
With the discovery that certain members of the polyester family crystallize very much more rapidly than poly(ethylene terephthalate) has come the development of more economical molding compositions with equivalent and, in many cases superior properties, with the complete elimination of the need for precise molecular weight control, the inclusion of nucleating agents, and the use of commercially unattractive molding cycles.
By way of illustration, poly(1,4-butylene terephthalate) resins crystallize very rapidly from the melt. These provide excellent molding compositions because they can be fabricated with moderate stock temperatures, low mold temperatures and rapid cycle times. Because of their highly crystalline nature, these resins are superior in chemical resistance, thermal stability and product appearance (they have a smooth, glossy finish). Such resins also have superior strength, stiffness, low friction and wear properties and good resistance to brittle fracture.
The poly(butylene terephthalate) resins can also be provided in reinforced and flame retardant embodiments.
One disadvantage of the poly(1,4-butylene terephthalate) resins resides in their significantly higher cost of manufacture -- in comparison with poly(ethylene terephthalate) -- mainly due to the need to employ more expensive raw materials.
It has now been discovered that the need to modify the molecular weight, include nucleating agents and lengthen the molding cycles with poly(ethylene terephthalate) resins and the economic disadvantages inherent in producing molding compounds in which poly(butylenephthalate) resins are the thermoplastic component, can be eliminated and overcome by using a novel, alloyed combination of the two polyester resins in the molding compositions.
In addition to the above advantages, parts molded from the alloyed combination often show properties superior to those of either of the components in their pure stage.
It is unexpected to obtain the above results and advantages because most polymers are incompatible in that they do not form homogeneous blends that stay permanently homogeneous. It is believed that the difficulty in homogenizing such resins into an acceptable blend increases with differences in molecular weight of the individual components, with increasing concentrations of low molecular weight components or with differences in morphology and/or rates of crystallization. Because poly(ethylene terephthalate) is known to crystallize only very slowly from the melt and poly(1,4-butylene terephthalate) is known to crystallize very rapidly from the melt, in view of the above, it would be entirely unexpected to find that blends of these two resins prove to be highly compatible both on the macro and molecular scale. In other words, these two polyester resins, which should be incompatible on the basis of the wide difference in their rates of crystallization, have, in fact, been discovered to form a stable alloy.
Molded parts containing poly(ethylene terephthalate) alloyed with poly(butylene terephthalate), unmodified, as well as reinforced, and also flame retardant embodiments, have excellent appearance, with no delamination between polymeric phases. The appearance of parts molded from the alloy containing significant amounts of poly(ethylene terephthalate) is indistinguishable from the excellent appearance of parts molded from compositions containing poly(1,4-butylene terephthalate) as the sole resinous component.
Other evidence for the formation of a true alloy between the poly(ethylene terephthalate) resin and the poly(1,4-butylene terephthalate) resin is provided by differential scanning calorimetric data. The crystalline melting points are found to change with the concentration of the constituents and, in certain cases, only one melting point is seen, the alloy having formed a eutectic or single crystal phase. These data will be exemplified hereinafter.
As the result of the present invention, compositions are provided with alloyed polyester resins, the compositions having economic advantages, without sacrifice in moldability, strength and heat distortion temperature and such compositions are commercially attractive because of their compatibility on a molecular scale.
Minor amounts of other polyesters or copolyesters can be included in the alloys. For example, a small amount of poly(1,4-dimethylolcyclohexane terephthalate) can be present. Or small amounts of other aromatic dicarboxylic acids such as isophthalic acid and naphthalene dicarboxylic acid or aliphatic dicarboxylic acids, such as adipic acid may be substituted for the terephthalic acid components. Small amounts of other diols, such as propane diol, or 1,4-dimethylol cyclohexane can replace the aliphatic diols.
It is an object of this invention to improve the moldability of poly(ethylene terephthalate) while improving the properties of poly(1,4-butylene terephthalate) and to provide compositions having many properties improved over those of compositions containing either resin alone.
A further object is to provide means to custom formulate compositions of poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) to obtain compositions having properties ranging between those obtained with compositions containing either resin alone.
Still another object is to provide compositions with substantially all of the properties of poly(1,4-butylene terephthalate), more economically than heretofore; and to provide compositions with economy approaching those containing poly(ethylene terephthalate) as the sole resinous component, but with improved properties.
It is a primary object of this invention to provide alloyed thermoplastic compositions comprising a poly(ethylene terephalate) and a poly(1,4-butylene terephthalate), wherein the disadvantages inherent in each of the resins are substantially overcome.